[clang] Fix crash on defaulted comparison recovery in Sema and AST (PR #210668)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Ok, I think this is true of any default comparison operator as you can repro with

```cpp
  struct B {
    friend bool operator==(const B&, const B&) = default;
    static_assert(B{} == B{});
  };
```

@Sirraide I _think_ the issue is that `DefineDefaultedComparison` needs to be deferred until the class definition is complete, but I'm unsure how in class `static_assert`ions work if they're dependent on the class def being completed (e.g. is @purnima-nlp going to have to do anything complex for first time contributor?)

https://github.com/llvm/llvm-project/pull/210668